(1) Field of the Invention
The invention relates to a fast disconnection device for surge arresters, specifically disc-shaped or flat varistors, comprising at least one element held under a mechanical preload as well as a point of separation in order to disconnect the surge arrester from the respective mains in the event of a thermal overload, wherein the point of separation includes contacts, the positions of which are variable relative to each other, and wherein one of these contacts is a fixed one.
(2) Description of Related Art
Disconnection devices of common efficient surge arresters with In>15 kA 8/20 μs including varistors for use in low-voltage mains generally comprise a soldered connection serving as thermal point of separation, a movable contact being under a mechanical preload, as well as a constricted region which adiabatically melts when reaching its melting integral. The soldered connection of the movable contact part constitutes the connection to the varistor contact and assumes the function of the thermal point of separation disconnecting the varistor from the mains when the varistor is overheated. Typical disconnection devices of this type are shown, for example, in DE 42 41 311 A1 or DE 38 05 889 A1.
In prior assemblies of this type the soldered connection connects two metal parts having a more or less high thermal conduction and quite a high thermal capacity, which is due to the fact that these metal parts have to control all electrical loads occurring in the operative range of the arrester. The soldered connection itself has to meet the requirements of the pulsed current carrying capacity. In addition, the soldered connection is permanently exposed to the mechanical load of the spring preload acting on the movable part of a corresponding lead.
An optimization of the essential functions of the point of separation, namely the desired disconnection from the varistor when it is heated, is thus not possible. Therefore, the solutions according to the prior art are forced to find compromises. As a result thereof, all solutions according to the prior art have in common that the thermal disconnection device has a considerable inertia, which is above all based on the thermal conductivity of the used materials, the necessary cross-sections of the contacts and the resulting great thermal capacity of the overall assemblies.
In the event of an overload, that is, when the varistor is heated as a result of long-lasting overvoltages caused by operation frequencies or as a result of aged varistors, the soldered connection of the disconnection device is heated only gradually due to the above-explained conditions. This means that even upon reaching a temperature critical for the varistor a certain time passes until the soldered connection has reached the necessary melting temperature. This time may be reduced by using solders with melting temperatures below the critical temperatures of the varistor. This is the reason why the varistor is frequently overloaded before a thermal disconnection takes place. However, an overloaded varistor can lead to undefined influences with respect to the resulting fault currents and, thus, to a plurality of different faults and further damages.
An arrester disconnection switch is known from EP 0 862 255 A1, according to which a mechanical switch disconnecting the varistor from the mains can be triggered by means of a fault current measurement and an evaluation. The disadvantageous properties involved by the usual contacting of varistors via a thermal, sensitive, but inevitably delayed-action soldering point can principally be avoided. The effort required for the detection of the fault current and the actuation of the switch as well as for the necessary evaluation is considerable, however.
Document DE 28 53 697 A1 describes a series connection comprised of a varistor and a switch. The thermal overcurrent release of the switch is not only heated by the fault current, but also directly by the heated varistor. Such an assembly, too, is constructively complicated and has the drawback that the release characteristic of the switch can be adapted to the requirements of the varistor to a limited extent only. The release system and the switch as a whole have to thermally and dynamically control pulsed currents in the range of several 10 kA. However, in order to obtain an optimum protection, the release should already be possible at a few mA. A direct heating of the bimetallic release of the switch by the varistor is helpful but, due to the aforementioned requirements, the release has a correspondingly great mass and thermal capacity counteracting a short response time.
Document WO 2004/064213 A1 shows an assembly in which a bimetal locks a switching contact. The bimetal is not flown through by the current. If the bimetal is indirectly heated as a result of the varistor being heated, the movement of the bimetal releases the locking and the varistor is disconnected from the mains. The indirect heating via the electrical connecting leads of the varistor, the geometrical arrangement, the material properties and the inevitably required dimensions of the bimetal likewise preclude, under the physical-constructive aspect, a very short response time until the contact is opened.
DE 36 32 224 A1 discloses a series connection of a switching contact and a varistor, wherein the expansion of an expanding substance is used to disconnect the varistor. The expanding substance is located directly on the varistor and, being applied to the full surface, is heated by the varistor. The expansion of the expanding substance in one direction corresponds the distance traveled by the switching contacts. The prolongation of the isolating distance is therefore not only limited, but is also very slow, so that the switching capacity of the isolating distance as a whole remains reduced. Also, the expansion of the expanding substance takes place with a delay so that a fast disconnection required in the event of a fault cannot be realized.
It becomes obvious that all solutions known from the prior art are only compromises between the possible, obtainable thermal sensitivity of the point of separation and the required current carrying capacity. As a rule, the thermal disconnection devices have a considerable inertia which is due to the good thermal conductivity of the used materials and the necessary large cross-sections for contacts along with a great thermal capacity resulting therefrom.